1. Field of the Invention
The present invention relates to a process for producing an activated carbon. More specifically, it relates to a process for producing an activated carbon, process which makes it possible to produce activated carbons whose smaller pores are closed selectively.
2. Description of the Related Art
Gasoline, an automotive fuel, is highly volatile so that it vaporizes in fuel tanks to turn into gasoline vapors when automobiles are driven or when they are parked under the scorching sun. The gasoline vapors are emitted into air eventually.
Accordingly, canisters are installed between fuel tanks and engines in order to inhibit the gasoline vapors from emitting to the outside of automobiles. The canisters adsorb the gasoline vapors with adsorbents.
Many canisters are provided with activated carbons as an adsorbent. In the canisters, the activated carbons adsorb the gasoline vapors which are generated in fuel tanks. The adsorbed gasoline vapors are desorbed or purged from the activated carbons as the revolving speed of engines increases. Then, the desorbed or purged gasoline vapors are introduced into exhaust pipes along with air which comes from the outside, and are burned therein.
Recently, it has been required for the canisters not only to adsorb the gasoline vapors in fuel tanks but also to adsorb gasoline vapors when gasoline is supplied. In other words, it has been required for the canisters to show furthermore improved gasoline-vapor adsorption performance.
Moreover, it has been required for the canisters to exhibit not only the adsorption performance but also proper desorption performance.
For example, when gasoline vapors are adsorbed onto conventional canisters and the adsorbed gasoline vapors are thereafter desorbed by means of air, the conventional canisters might suffer from the problem that the adsorbed gasoline vapors reside on activated carbons. When the gasoline vapors reside on activated carbons, the residual components desorb from the activated carbons as the daytime temperature increases, and have been emitted eventually to the outside of automobiles. In order to inhibit the residual components from leaking to the outside, various countermeasures, such as providing the conventional canisters with a sub-canister, have been taken, but have resulted in causing the other problem, the increment of cost involved.
In general, activated carbons have been produced by carbonizing raw materials followed by carrying out the activation. In the production of activated carbons, the activation is a process which develops pores and controls the pore diameter or opening diameter of the pores. It is required for activated carbons applied to canisters to have pores, which exhibit such a large pore diameter that falls in a range of from 20 to 50 Å (i.e., 2 to 5 nm), in order to adsorb and desorb gasoline vapors. Such large pores are made by advanced activation processes or chemical activation processes which are carried out under severe conditions than those of usual activation processes as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2000-313,611, Japanese Examined Patent Publication (KOKOKU) No. 1-52,324 and Japanese Unexamined Patent Publication (KOKAI) No. 63-30,308, for instance.
When the advanced activation processes or chemical activation processes are carried out, it is possible to produce activated carbons provided with larger pores. However, such activated carbons have exhibited a broad pore diameter distribution. To put it differently, the activated carbons comprise a large number of smaller pores as well whose opening diameter or pore diameter is small. The aforementioned residual components reside in such smaller pores whose pore diameter is small. Since smaller pores whose pore diameter is small exhibit higher absorptivity than that of larger pores whose pore diameter is large, adsorbed components are less likely to desorb from smaller pores. That is, in canisters, adsorbed components, which cannot desorb from and have remained in smaller pores having a small pore diameter, have caused the leakage of gasoline vapors when the ambient temperature increases.